Light guide

ABSTRACT

A flexible light guide comprises a flexible tube of a transparent thermoplastic containing a transparent liquid having a higher refractive index than the thermoplastic. The tubing may be of a polymer or copolymer of 4-methyl pentene-1 or any clear plastic tubing.

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Cass 1 June 19, 1973 SUBSTITUTE FOR MISSING OR LIGHT GUIDE 2,827,8253/1958 White 350/96 x [75] Inventor: Michael Cass, Welwyn Garden City,gggi 2 England FOREIGN PATENTS OR APPLICATIONS I73] Assignee: ImperialChemical Industries,

. 1,037,498 7/1966 Great Britain 350/96 R Limited, London, England Feb.16, 1970 OTHER PUBLICATIONS Tove, Article in The Review of ScientificInstruments" Vol. 27, No. 3 March, 1956 pgs. 143-146. Kapany FiberOptics Textbook published 1967 pgs.

{22] Filed:

[21} Appl. No.:11,431

[30] Foreign Application Priority Data 298-302, 304, 310 & 3] 1,

Mar. 13, 1969 Great Britain 13,205/69 Primary Examiner-David H. RubinI52] US. Cl. 350/96 R, 350/312 Alt0rneyCushman, Darby & Cushman I51]Int. Cl. G02!) 5/14 [58] Field Of Search 350/96 R, 96 B, 96 W6, [5-7]ABSTRACT A flexible light guide comprises a flexible tube of a [56]References Cited transparent thermoplastic containing a transparentliquid having a higher refractive index than the thermo- UNITED STATESPATENTS plastic. The tubing may be of a polymer or copolymcr 3,363,1741/1968 Hudson et a1. 350/96 R X of Lmehy] 1 or any d l i bi 3,492,4931/1970 Herriot et a1.

3,434,776 3/1969 Kern 350/96 9 Claims, 1 Drawing Figure N IJI-LM LIGHTGUlDE The present invention relates to light guides and in particular toflexible light guides.

Light guides are used to pipe light and flexible light guides areavailable which can be used to illuminate inaccessible areas. Theselight guides comprise a bundle of fibres of light transmitting materialcontained in a flexible sheath ofa suitable material, for example,polyethylene. In such light guides, light enters the fibre at one end,and passes along the length of the fibre either directly, or in azig-zag path by total internal reflection of the light within the fibre.These fibres have conventionally been glass fibres, and light guidesusing glass fibres are expensive to produce. As an alternative to glassfibres, it has been proposed to use fibres of acrylic polymers,typically polymethylmethacrylate. Whilst such fibres are more readilyproduced than glass fibres, in forming fibre light guides of eithertype, since the arrangement and alignment of the fibres is an operationrequiring great care and therefore time, the production of such lightguides is an expensive procedure.

According to the present invention there is provided a flexible lightguide comprising a flexible tube of a transparent thermoplasticmaterial, which tube is closed at either end with a plug of atransparent material; and contains a transparent fluid having arefractive index which is greater than that of the thermoplasticmaterial.

In this manner there is obtained a light guide which is simple andinexpensive to produce. Surprisingly, most thermoplastic materialspresently available either are not sufficiently flexible or do not havethe requisite transparency for use in accordance with the presentinvention. Thus, polyethylene and polypropylene can be formed intoflexible tubing but these materials are not sufficiently transparent tobe used in accordance with the present invention. Conversely, materialssuch as acrylic polymers, for example, polymethylmethacrylate, orpolystyrene, have sufficient transparency to function as a light guide,but are insufficiently flexible to give a suitable flexible light guide.Clear polyvinyl chloride is both flexible and transparent, but has ahigh refractive index and this means there are few satisfactory liquidsavailable for use with this polymer. We have found, however, thatpolymers of 4-methy1 pentene-l can be formed into tubing which issufficiently flexible and also transparent, and furthermore that therefractive index ofthis polymer is less than that of polyvinyl chlorideand thus a wider range of suitable liquids is available. It is a furtheradvantage of 4-methyl pentene-l polymers compared to polyvinyl chloride,that using a given liquid in the thermoplastic tube, the critical anglefor total internal reflection is less with the 4- methyl pentene-lpolymer than with polyvinyl chloride, thereby permitting the tubing tobe bent more sharply with the same loss of light through the sides ofthe tube. A further advantage of 4-methyl pentene-1 polymer is that,like other polyolefines, it possesses good chemical resistance to anumber of transmitting fluids.

The plug of transparent material at either end of the tube may be formedof any suitable transparent material. Although for compatibility, itmight be preferred that the plug is of the same material as the tube,materials such as acrylic polymers or glass can also be used for the endplugs. One ofthe end plugs is preferably secured into position beforethe liquid is introduced into the tube and the other end plug is thenfitted into position when the tube has been filled with the liquid. Theplug may be sealed to the tube by heat sealing but, once the liquid hasbeen introduced into the tube, it is preferred to secure the second plugusing a suitable adhesive or cement.

If desired, to produce a coloured light, the liquid in the light guidemay be coloured, this conveniently being effected by adding anappropriate coloured soluble dyestuff to the liquid. Alternatively, acoloured filter can be located at one end of the light guide to give thedesired colour, which permits several different colours to be obtainedusing one light guide and a number of movable coloured filters.

Flexible light guides in accordance with the invention can be used toilluminate several instruments on an instrument panel in a car oraircraft using only one light source, a number oflight guides directinglight from the source to the appropriate instrument on the panel. Al-

ternatively, particularly using a tube formed of a copolymer of 4-methylpentene-1 with a high (for example 10 percent by weight) content ofalinear olefine co monomer, such as hexene-l, octene-l or deccne-l, aflexible light guide is obtained which has a good heat resistance (themelting point of 4-methyl pentene-l polymers is about 240 C) and can besterilised and thus used in surgical applications or in industry wherehigh temperatures can be experienced. Very satisfactory flexible lightguides may be formed using copo1y mers of 4-methyl pantene-l and from 4to 6 percent by weight of a linear olefine comonomer, particularlydecene-l.

As indicated herein, polymers, including copolymers, of 4-methylpentene-1 are particularly suitable for use in accordance with thepresent invention, and some suitable polymers are described in BritishPat. Nos. 942,297; 968,935; 1,001,801; 1,014,886 and 1,085,914.Particularly suitable 4-methyl pentene-1 polymers are sold under theTrade Mark TPX-RT methyl pentene polymers by Imperial Chemicallndustries Limited.

It will be appreciated that the liquid used must be compatible with thepolymer and a number of liquids which are generally suitable for usewith 4-methyl pentene-l polymers, together with the refractive index ofthe liquid, are set out in the following table:

Liquid Refractive Index Benzyl alcohol 1.54 Nitrobenzcne 1.556 LinseedOil 148 Chlorohcnzene 1.525 Caster Oil 1.477 Camphor 1.546 Sikiconefluid 1.49

sold by General Electric as SF. 1017 Chlorobenzene diffuses through4-methy1 pentene-1 polymers and thus is not completely suitable for usewith such polymers. Many silicones have a refractive index in the range1.40 to 1.43 and are accordingly unsuitable since this is less than therefractive index of 4- methyl pentene-1 polymers. By way of comparison,the refractive index of TPX is 1.465 and that of clear polyvinylchloride is 1.543. lt will be seen that few of the liquids noted in thetable have a higher refractive index than polyvinyl chloride and thosethat do have a higher refractive index have only slightly greaterrefractive indices and thus will give little total internal reflectionwithin the tube. For this reason, 4-methyl pentene-l polymers are muchmore suitable than polyvinyl chloride for use in accordance with thepresent invention. Of the liquids set out in the table, benzyl alcoholhas been found to be particularly good in respect ot'giving high lighttransmission.

EXAMPLE A 4 foot long tube of TPX-RT having a A inch external diameterand a wall thickness of 0.020 inches was filled with benzyl alcohol.Using a 12 volt, watt festoon bulb as the light source, a beam of lightwas emitted from the other end of the tube when bent through an angle of360. A similar effect was obtained when caster oil was used as theliquid medium, although a smaller quantity of light was transmitted.When water was used as the liquid, no light was transmitted through thebent tube. The refractive index of water is 1.333, which issubstantially less than that of TPX'.

The experiment was repeated using similar tubes formed from copolymersof 4-methyl pentene-l with 4 percent or 6 percent by weight of decene-l.The liquid used was benzyl alcohol. These tubes are more flexible thanthe TPX-RT tube and are also less susceptible to stress whitening oryielding. Otherwise, a similar effect as obtained as with the TPX-RTtube.

The accompanying drawing is a sectional view of a light guide inaccordance with the present invention being used to "bend" light throughan arc of 270.

A tube 1 of TPX-RT is sealed at either end by a transparent end plug 2and 3, also of TPX'-RT. The sealed tube 1 contains caster oil (4) as theliquid filling the tube 1. The tube is bent through an arc of 270 C, anda light source 5 is located adjacent to the end plug 2. Light from thesource 5 passes through the plug 2 and into the caster oil 4. Most ofthe light undergoes total internal reflection at the inner surface ofthetube 1 and ultimately emerges from the tube 1 through the end plug 3, asa beam of light shown diagrammatically at 6. The tube 1 is flexible andcan be moved about so that the light beam 6 emerging from the end plug 3may be aimed in any desired direction. Not all the light is internallyreflected at the inner surface of the tube 1, some passes straightthrough the tube 1 and is lost. The main losses of light occur in theinitial section of the passage of the light through the tube 1, and alsoa the initial curved section of the tube. These losses can be reduced bythe use ofa liquid having a higher refractive index than caster oil,such as benzyl alcohol. A further source of light losses is from thepresence of irregularities on the inner surface of the tube, and for thebest results the internal surface ofthe tube 1 has to be absolutelysmooth and free from any flow defects.

If desired, the tube 1 may be enclosed along the whole of its length byan opaque flexible outer sheath (not shown), thus'leaving exposed onlythe end plugs 2 and 3 through which light enters and leaves the tube.Any suitable material may he used for this outer sheath. which canconveniently be a flexible black polythene sheath.

l claim:

1. A flexible light guide consisting essentially of a tube of atransparent flexible thermoplastic material selected from transparentpolyvinyl chloride and 4- methyl pentene-l polymers, a plug ofatransparent material at either end of the tube to close the tube, and atransparent fluid within the tube filling it, wherein said.

fluid has a refractive index greater than that ofthe thermoplasticmaterial and is selected from nitrobenzene, camphor, linseed oil,chlorobenzene, castor oil, and benzyl alcohol.

2. The light guide of claim 1 wherein the plug of transparent materialis selected from an acrylic polymer, glass, and the same material as theflexible tube.

3. The light guide of claim I wherein the fluid in the tube is a liquidwhich is coloured with a dyestuff soluble in the liquid.

4. The light guide of claim 1 wherein the tube is formed of clearpolyvinyl chloride and the fluid is selected from nitrobenzene andcamphor.

5. The light guide of claim 1 wherein the tube is formed of a 4-methylpentcne-l polymer and the fluid is selected from camphor and benzylalchol.

6. The light guide of claim 5 wherein the 4-methyl pentene-l polymer isa copolymer with a linear olcfine.

7. The light guide of claim 6 wherein the 4-methyl pentene-l polymer isa copolymer with a linear olefin selected from hexenc-l, octene-l ordecenel.

8. The light guide ofclaim 6 wherein the copolymer contains up to IDpercent by weight of the linear comonomer.

9. The light guide of claim 8 wherein the copolymer contains from 4 to 6percent by weight of the linear comonomer.

* x x x

2. The light guide of claim 1 wherein the plug of transparent material is selected from an acrylic polymer, glass, and the same material as the flexible tube.
 3. The light guide of claim 1 wherein the fluid in the tube is a liquid which is coloured with a dyestuff soluble in the liquid.
 4. The light guide of claim 1 wherein the tube is formed of clear polyvinyl chloride and the fluid is selected from nitrobenzene and camphor.
 5. The light guide of claim 1 wherein the tube is formed of a 4-methyl pentene-1 polymer and the fluid is selected from camphor and benzyl alchol.
 6. The light guide of claim 5 wherein the 4-methyl pentene-1 polymer is a copolymer with a linear olefine.
 7. The light guide of claim 6 wherein the 4-methyl pentene-1 polymer is a copolymer with a linear olefin selected from hexene-1, octene-1 or decene-1.
 8. The light guide of claim 6 wherein the copolymer contains up to 10 percent by weight of the linear comonomer.
 9. The light guide of claim 8 wherein the copolymer contains from 4 to 6 percent by weight of the linear comonomer. 